Principal objectives of this study are to obtain a total of 80 primary and derived variables on 168 normal women at 5 year intervals. The measurements consist in comprehensive calcium kinetic, calcium metabolic, and physiological measurements as well as radiogrammetric and photon absorptiometric measurements of bone mass. The purposes are: 1. To obtain comprehensive longitudinal data on calcium metabolism, bone metabolism, and related physiological functions in a group of untreated women from the perimenopausal period to the median onset time of osteoporotic symptoms; 2. Thereby to provide an exhaustive physiological profile both on women who develop symptomatic osteoporosis, and on those who do not, allowing characterization of the differences between them and of the factors influencing the differences; 3. Thereby permitting better identification of the osteoporosis-prone subset and providing both a rational basis for prophylactic treatment and the identification of subjects who should be its target; 4. And finally, to characterize cross-sectional interrelationships between major physiological variables during this key era of osteoporosis development.